Chapter 21

Question 1

21-1

Answer 1

The strength of a particle’s electrical interaction with objects around it depends on its electric charge (usually represented as q), which can be either positive or negative. Particles with the same sign of charge repel each other, and particles with opposite signs of charge attract each other.

Question 2

21-1

Answer 2

An object with equal amounts of the two kinds of charge is electrically neutral, whereas one with an imbalance is electrically charged and has an excess charge.

Question 3

21-1

Answer 3

Conductors are materials in which a significant number of electrons are free to move. The charged particles in nonconductors (insulators) are not free to move.

Question 4

21-1

Answer 4

Electric current i is the rate dq/dt at which charge passes a point: i = dq/dt

Question 5

21-1

Answer 5

Coulomb’s law describes the electrostatic force (or electric force) between two charged particles. If the particles have charges q1 and q2, are separated by distance r, and are at rest (or moving only slowly) relative to each other, then the magnitude of the force acting on each due to the other is given by F = (1 / 4piE0) (|q1| |q2|) / r^2 (Coulomb’s law), where E0 = 8.85 * 10^-12 N * m^2/C^2 is the permittivity constant. The ratio (1 / 4piE0) is often replaced with the electrostatic constant (or Coulomb constant) k = 8.99 * 10^9 N * m^2/C^2.

Question 6

21-1

Answer 6

The electrostatic force vector acting on a charged particle due to a second charged particle is either directly toward the second particle (opposite signs of charge) or directly away from it (same sign of charge).

Question 7

21-1

Answer 7

If multiple electrostatic forces act on a particle, the net force is the vector sum (not scalar sum) of the individual forces.

Question 8

21-1

Answer 8

Shell theorem 1: A charged particle outside a shell with charge uniformly distributed on its surface is attracted or repelled as if the shell’s charge were concentrated as a particle at its center.

Question 9

21-1

Answer 9

Shell theorem 2: A charged particle inside a shell with charge uniformly distributed on its surface has no net force acting on it due to the shell.

Question 10

21-1

Answer 10

Charge on a conducting spherical shell spreads uniformly over the (external) surface.

Question 11

21-2

Answer 11

Electric charge is quantized (restricted to certain values).

Question 12

21-2

Answer 12

The charge of a particle can be written as n*e, where n is a positive or negative integer and e is the elementary charge, which is the magnitude of the charge of the electron and proton ( ~= 1.602 * 10^-19 C).

Question 13

21-3

Answer 13

The net electric charge of any isolated system is always conserved.